In a large pot of boiling salted water, cook pasta according to package instructions; drain well.Melt butter in a large skillet over medium high heat. Add shrimp, garlic and red pepper flakes. Cook, stirring occasionally, until pink, about 2-3 minutes. Stir in Chicken Stock and lemon juice; season with salt and pepper, to taste. Bring to a simmer; remove from heat and stir in pasta, lemon zest and parsley.Serve immediately, garnished with Parmesan, if desired.